1. Field of the Invention
The present invention relates to an organic electroluminescent device and more particularly, to an active matrix organic electroluminescent device and a method of fabricating the same.
2. Discussion of the Related Art
In general, an organic electroluminescent device (OLED) emits light by injecting electrons from a cathode and holes from an anode into an emission layer, combining the electrons with the holes, generating an exciton, and transitioning the exciton from an excited state to a ground state. Compared to a liquid crystal display (LCD) device, an additional light source is not necessary for the OLED to emit light because the transition of the exciton between the two states causes light to be emitted. Accordingly, the size and weight of the OLED can be reduced. The OLED has other excellent characteristics such as low power consumption, superior brightness, and fast response time. Because of these characteristics, the OLED is a promising display for next-generation consumer electronic applications such as cellular phones, car navigation systems (CNS), personal digital assistants (PDA), camcorners, and palmtop computers. Moreover, since fabricating the OLED is a simple process with a few processing steps, an OLED is cheaper to produce than a LCD device.
Two different types of OLEDs exist: passive matrix and active matrix. While both the passive matrix OLED and the active matrix OLED have a simple structure and are formed by a simple fabricating process, the passive matrix OLED requires a relatively high amount of power to operate. In addition, the display size of a passive matrix OLED is limited by its structure. Furthermore, as the number of conductive lines increases, the aperture ratio of a passive matrix OLED decreases. In contrast, active matrix OLEDs are highly efficient and can produce a high-quality image for a large display with relatively low power.
FIG. 1 is a schematic cross-sectional view of an OLED according to a related art. In FIG. 1, an array element 14 including a thin film transistor (TFT) T is formed on a first substrate 12. A first electrode 16, an organic electroluminescent layer 18, and a second electrode 20 are formed over the array element 14. The organic electroluminescent layer 18 may separately display red, green, and blue colors for each pixel region. A second substrate 28 faces the first substrate 12 and is spaced apart from the first substrate 12.
The first and the second substrates 12 and 28 are attached to each other with a sealant 26. The OLED is encapsulated by attaching the first substrate 12 to the second substrate 28. The second substrate 28 includes a moisture absorbent material 22 to eliminate moisture and oxygen that may penetrate into a capsule of the organic electroluminescent layer 18. After etching a portion of the second substrate 28, the etched portion is filled with the moisture absorbent material 22 and the filled moisture absorbent material is fixed by a holding element 25.
FIG. 2 is an equivalent circuit diagram of the OLED according to the related art. In FIG. 2, a gate line 36 crosses a data line 49, and a switching element TS at a crossing of the gate line 36 and the data line 49 is connected to the gate line 36 and the data line 49. A driving element TD electrically connects the switching element TS to an organic electroluminescent diode DEL. A storage capacitor CST is formed between a driving gate electrode 34 and a driving source electrode 52 of the driving element TD, as the driving element TD is a positive type transistor. The organic electroluminescent diode DEL is connected to a power line 62, and the driving drain electrode may be connected to an anode of the organic electroluminescent diode DEL.
When a scan signal of the gate line 36 is applied to a switching gate electrode 32 of the switching element TS, an image signal of the data line 49 is applied to the driving gate electrode 34 of the driving element TD through the switching element TS. The current density of the driving element TD is modulated by the image signal applied to the driving gate electrode 34. As a result, the organic electroluminescent diode DEL can display images with gray scale levels. Moreover, because the image signal stored in the storage capacitor CST is applied to the driving gate electrode 34, the current density flowing into the organic electroluminescent diode DEL is uniformly maintained until the next image signal is applied, even when the switching element TS is turned off. The switching element TS and the driving element TD can be a polycrystalline silicon TFT or an amorphous silicon TFT. The process of fabricating an amorphous silicon TFT is simpler than the process for a polycrystalline silicon TFT.
FIG. 3 is a schematic cross-sectional view illustrating a switching element and a driving element including an amorphous TFT for one pixel region of an OLED according to the related art. In FIG. 3, a gate line 36 is formed on a substrate 30 in a first direction, a data line 49 crosses the gate line 36 in a second direction to define a pixel region P, and a power line 62 is arranged in parallel to the data line 49 and crosses the gate line 36. A switching element TS adjacent to the pixel region P is connected to the gate and data lines 36 and 49. A driving element TD is connected to the switching element TS. In addition, the switching element TS includes switching gate electrode 32, switching semiconductor layer 56, switching source electrode and drain electrode 48 and 50. The driving element TD includes a driving gate electrode 34, a driving semiconductor layer 58, a driving source electrode and a driving drain electrode 52 and 54. Specifically, the driving gate electrode 34 is connected to the switching drain electrode 50, the driving source electrode 52 is connected to the power line 62, and the driving drain electrode 54 is connected to a first electrode 66 of the organic electroluminescent diode DEL (of FIG. 2). The switching semiconductor layer 56 and the driving semiconductor layer 58 may be formed of amorphous silicon.
The amorphous silicon driving TFT should have a large width to length ratio (W/L ratio) in order to drive the organic electroluminescent diode DEL (of FIG. 2). In this case, a size of the driving element TD is much larger than a size of the switching element TS in order to supply enough current to the organic electroluminescent diode DEL.
Accordingly, to obtain a large width to length ratio (W/L ratio), the driving source electrode and drain electrode 52 and 54 include first driving source electrode and drain electrode 52a and 54a along the first direction, and second driving source electrode and drain electrode 52b and 54b extending from the first driving source electrode and drain electrode 52a and 54a along the second direction such as a finger shape, respectively. Here, the second driving source electrode 52b alternates with the second driving drain electrode 54b. 
FIG. 4 is a schematic plan view showing a driving element of an OLED according to a first example of the related art.
In FIG. 4, a driving element TD includes a driving gate electrode 34, a driving semiconductor layer 58 over the driving gate electrode 34, a driving source electrode 52 on the driving semiconductor layer 58 and a driving drain electrode 54 spaced apart from the driving source electrode 52 on the driving semiconductor layer 58. Although not shown, the driving semiconductor layer 58 includes a driving active layer of an intrinsic amorphous silicon and a driving ohmic contact layer of a doped amorphous silicon on the active layer. An exposed portion of the driving active layer between the driving source electrode and the drain electrode 52 and 54 acts as a channel in which electrons or holes pass through. As the width to length ratio (W/L ratio) increases the on current characteristic correspondingly improve. Therefore, to obtain this advantage, the driving source electrode and drain electrode 52 and 54 are formed as a plurality of finger shapes. At this time, the driving gate electrode 34 under the driving source electrode and drain electrode 52 and 54 is formed with an opening portion OP to minimize an overlap portion between the driving gate electrode 34 and the driving source electrode and drain electrode 52 and 54. More specifically, this structure is utilized for reducing parasitic capacitance due to the overlapping portion.
Hereinafter, it will be explained about the structure in accordance with the driving gate electrode 34 and the driving source electrode and drain electrode 52 and 54 referring to a specific numerical value.
When a channel length L defined as a distance between the driving source electrode 52 and the driving drain electrode 54 is about 6 micrometers and an overlapping width between the driving gate electrode 34 and the driving source electrode and drain electrode 52 and 54 is about 3 micrometers, a minimum width of the driving gate electrode 34 should be about 12 micrometers. Accordingly, the distance between the plurality of driving gate electrode 34 patterns adjacent to each other is about 6 micrometers and the distance between the driving source electrode 52 and the driving drain electrode 54 is about 6 micrometers. In addition, the widths of the driving source electrode 52 and the driving drain electrode 54 should be at least about 12 micrometers, respectively. Here, outermost portions of the driving source electrode 52 overlapping outermost portions of the driving gate electrode 34 correspond to about 6 micrometers.
Consequently, a width of the driving element TD of FIG. 4 can be calculated as follows:The width of the driving source electrode 52: (6 micrometers×2)+12=24 micrometersThe width of the driving drain electrode 54: 12 micrometers×2=24 micrometersThe total channel length L of the driving element TD: 6 micrometers×4=24 micrometersAccordingly, the width of the driving element TD: 24 micrometers×3=72 micrometers
Consequently, when the driving source electrode and drain electrode 52 and 54 are formed as having a finger shape and the driving gate electrode 34 is formed with the opening portion OP, the width of the driving element TD may be about 72 micrometers. A driving element TD having a ring shape is suggested to enlarge the width to length ratio (W/L ratio) as another example of the related art.
FIG. 5 is a schematic plan view showing a driving element of an OLED according to a second example of the related art.
In FIG. 5, a driving element TD includes a driving gate electrode 80, a driving semiconductor layer 82 over the driving gate electrode 80, a driving source electrode 83 and a driving drain electrode 86 spaced apart from the driving source electrode 83 on the driving semiconductor layer 82. Here, the driving gate electrode 80 has a first ring shape, and the driving source electrode 83 has a second ring shape overlapping outermost portion of the first ring shape of the driving gate electrode 80. The driving source electrode 83 surrounds the driving drain electrode 86 having an elliptical shape covering an opening portion OP of the driving gate electrode 80.
Also, this structure should be formed to obtain a large width to length ratio (W/L ratio), so the ring-type driving element is manufactured as an excessive size in comparison to the switching element TS (of FIG. 3). Therefore, since a size of the driving element TD occupies a significant area in the pixel region P (of FIG. 3), the display region is reduced. Consequently, it is difficult to manufacture an OLED having high aperture ratio and high resolution.